The invention relates to an optical transmission system comprising a number of transceivers which are mutually connected by means of a network and are provided with a transmitter, a receiver and a local oscillator, and also a frequency control device, connected to the local oscillator or a second oscillator, for mutually locking the frequency of the additional signal originating from the second oscillator and the frequency of the output signal of the local oscillator, the receiver being provided with an input for feeding at least a portion of the additional signal. Such a system is disclosed in European Patent Application 0 474 921.
This known optical transmission system comprises a first and second group of transceivers which are mutually coupled by means of a network, in particular a passive optical transmission star.
A complication of the coherent detection used in the known transmission system is the frequency fluctuation of the various lasers, such as the transmission lasers and the local oscillator lasers. In the case of a single coherent link, this problem could readily be eliminated by deriving from the received signal a control signal with which the frequency of the local oscillator is adjusted. This problem is less easy to eliminate in a transmission system having transmitters which are sited at a distance from one another but which have also to be stabilised in frequency.
In the transmission system according to the European Patent Application 0 474 921, an additional signal is used, the frequency of said additional signal and the frequency of the output signal of the local oscillator being mutually locked at a fixed frequency spacing. Used as additional signal is at least a portion of the signal from the transmitter oscillator or transmission laser. In order to obtain the frequency locking mentioned, a control signal has to be derived and fed to a frequency control device which is able to stabilise the frequency of the transmitter.
As a first solution, European Patent Application 0 474 921 proposes feeding a portion of the transmitter signal and a portion of the output signal of the local oscillator to an additional receiver whose output signal controls the frequency control device, which in turn adjusts the frequency of the transmitter.
According to another solution provided in the European Patent Application, the control signal is derived by means of a filter. In this solution, the coherent receiver has to recognise which signal is the incoming signal and which is its own transmitter signal. This makes a "cold start" of the system more complicated.
Summarised briefly, both solutions require additional expensive optical or electrical components for deriving the control signal.